Arthur: Really Father, I don't want to hear the details.
The dragon's laughter. Maybe it was just the juxtaposition of a character who is normally so bleak and solemn practically rolling on the floor laughing, but something about his laughter is just so infectious.
"The Tears Of Uther Pendragon"
Arthur: Would you like me to show you how to use the bucket?
Also from that episode:
Arthur: Describe Dollop Head.
Merlin: In two words? *Arthur nods* 'Prince Arthur.'
This scene in which Arthur and Merlin argue like an old married couple and M Erlin snarks at Arthur while Arthur tries to throw things at him.
The guard with the assault rifle in the background...which is pointed at Uther.
"You were CC'd on the email!!"
"Goblin's Gold"... almost all of it. Especially the parts where Gwen, Morgana, and Uther are farting loudly while court is held, Arthur's reaction to his ears getting scratched, and the "do girls even fart?" conversation between Merlin and Gwen.
Uther: The Council is dismissed!
[Gwen and Morgana fart again as the Council remains dumbstruck]
[everyone rushes out]
And also Arthur with donkey ears attempting to explain what happened to him while only able to bray, and Uther being struck bald (which is also an in-universe example - Merlin cracks up as soon as he's out of sight and earshot).
Not to mention Goblin-Gaius repeatedly slapping Uther's head.
Really, everything about Old!Merlin is a CMOF, especially noticeable in a rather bleak episode.
Old!Merlin: "I am DRAGOOOOOOON! ...the great!" "Well, I really wouldn't do THAT, 'cause if you do THAT... you'll never discover my plaaaan...." "EXACTLY, THAT'S WHY YOU CAN'T RUN ME THROUGH!" "I've heard how you... mistreat your servants!" and Arthur with, "...You're sentenced to death, it's not like we can execute you twice..."
Old!Merlin in 4x03 as well:
Old!Merlin: "Questions, so many questions, for once in your life would you just DO WHAT YOU'RE TOLD!"
Also in "The Wicked Day", Old Merlin insults Arthur even more, and gets him to give him a piggy back, down to dig his feet in as a "hurry up" gesture.
The keys scene, in which Merlin tries to steal Arthur's keys using magic. Arthur hears them jingling, and starts following the sound around the room, looking more and more ridiculous as Merlin moves them around so they're always behind him... and watches with a completely straight face.
Uther trying to talk sense into Arthur in Sweet Dreams. He repeatedly looks incredulously at Merlin for any sort of explanation of his son's behavior, and eventually throws his hands up and just walks away.
The Witchfinder. This exchange:
Witchfinder: My methods are infallible, my findings incontestable. The facts point to one person and once person alone. The boy, Merlin! *everyone gasps in fear*
Arthur: (completely deadpan, with as much skepticism as can be put into one word) Merlin. You can't be serious.
While camping in the Perilous Lands:
Merlin: What was that..?
*more horrible screaming*
Gwaine: A big pheasant.
Merlin: That sounded like three...pheasants.
In The Lady of The Lake while Merlin is stealing a dress
Arthur: So long as you do a decent dayís work Merlin, thatís all I care about.
Arthur's completely dignified response to Gaius's presence in his bed-chamber, even though he's stark naked in front of a rather delighted Gwen with only a pillow for modesty. There's a good chance he looks back on the whole experience as a particularly vivid dream.
Morganas Oh Crap as she finds 'Emrys' (Old Merlin) searching her hut. And Merlin simply shrugging and continuing.
In "Lamia," we have banter!
Arthur: So... Merlin... being saved by a woman, that really can't feel good.
Merlin: Feels a lot better than being dead.
Arthur: Does it? Being saved by a woman... or dying... I'd have to think about that one.
Merlin: Well don't think too hard, I wouldn't want you to hurt yourself.
Any scene with simpleton!Arthur is amazing. Particularly once Merlin starts getting exasperated with him or taking advantage of the situation to get out of his usual duties.
This deserves special mention (it is paraphrased, mind you):
Tristan: And we couldn't be caught by Camelot's idiot of a king.
(The camera pans to Arthur, who is knocking on a tree with one ear pressed against it.)
Arthur nonchalantly saying of Excalibur: "Hey, this thing's not bad!"
The WTF look on Arthur's face when Merlin starts juggling for Queen Annis.
This quote from Arthur's Bane Part 2
Merlin: I'll go lure the dragon away.
Arthur: I know you're stupid, Merlin, but not that stupid.
Merlin: No, really I'm that stupid and if you don't believe me, watch. *runs off*
In The Death Song of Uther Pendragon, Arthur speaks sternly to Merlin while holding a spoon up.
Merlin: You're threatening me with a spoon? [cut to hallway, a loud smack] OWW!
At the end of the episode, Merlin smacks Arthur upside the head hard with a leather glove.
Arthur: What the hell was that?
Merlin: ... was um... horseplay.
Merlin telling Arthur that "Some people think he's an arrogant ass." When asked who, he doesn't answer.
From The Dark Tower, Gwaine breaks up the tension every chance he gets.
When Leon and Percival wake up from nightmares, the rest of the group looks horrified. Gwaine intercedes with this:
Gwaine: I was dreaming about a cheese that tasted of apple pie. Anyone else have that dream? Well, you're missing out.
While in the Impenetrable Forest, the knights are hacking their way through. Gwaine eventually gets tired of it and starts hacking crazily in a circle. Cut to Percival, Merlin, and Arthur all looking at him worriedly.
From that same scene, Percival breaks his sword on a vine. He looks at it and throws it away.
Dragoon in A Lesson in Vengeance insulting his way past the guards in his usual style.
And when he unleashes his irritation in a shouting match with the cook. Then gets bored and knocks her out with a pan.
Dragoon: Should have done that years ago.
In With All My Heart, Arthur is hesitant to trust any sorceror since one killed his father. Gaius assures Arthur this one is different...she's a woman. Cue a Death Glare from Merlin!
Colin Morgan: Iím really popular around here. These guys really really like me as you can see on Bradleyís face.
Bradley James: Yeah.
Colin Morgan: Iíve heard thereís some really weird wildlife around here.
Bradley James: Oh I thought I saw something in the water earlier.
Colin Morgan: Really?
Bradley James: Maybe check it out. See if you can see anything.
Colin Morgan: I donít know. I mean I saw a cave spider earlier-(turns camera and sees the duck) WHAT THE HELL IS THAT?! (pause) What is that... Itís... a... itís a red-eyed DUCK. It's a red-eyed duck in the water.
Bradley James: Carefully. It will attack you.
Colin Morgan: Oh no. Itís floating towards us.
Bradley James: Itís coming straight for us.
Colin Morgan: Quick! Letís go!
Bradley James: Itís coming straight at us. Its got that evil look in its eye! (he falls on the floor)
Colin Morgan: Bradley? Bradley? Are you okay?
Amongst the best bits are when, while shopping in a french supermarket, Bradley James rants about not having a fridge, which is followed by this:
Anthony Head: You've gone red.
Bradley James: Yeah, cause that's how angry I am about not having a fridge!
Anthony Head: Why are you angry at not having a fridge?
Bradley James: Cause I can't drink milk!
Anthony Head: Oh. * beat* I've told you, milk's not that good for you.
As well as:
Bradley James: I have the magic phrase: * sings* 'Je ne comprends pas, je suis Anglais, ya-hey!'
* the entire Amusement Park video, to be honest. Not least of all for hearing Colin Morgan's potty mouth, or Anthony Head's "Splish Splesh" in an uncannily accurate French accent
Bradley James: Yes, I AM. If our names were actually representations of who we are, I'd be called Angel, and you'd be called... hussy!
Colin Morgan: See this is what is going on, he just bullies me on set and off set, it's just... it's like a constant thing. Since we've come to France, it's got really bad.
Bradley James: Oh, has it?
Colin Morgan: Yeah.
Bradley James: Oh, it's really bad, is it? Oh, I'm so sorry. I'm so sorry that you're being bullied, ''MERlin''. (Colin shakes head and leaves) Right, everyone, okay? Everyone looooves Colin 'oh he's so nice, he's so innocent', you know, 'awww Colin, awww'. 'Oh, Bradley, don't worry about Bradley, he's fine, yeah, you know, whatever. Colin awww, look after Colin, uuh'. If I weren't here to give Colin Morgan a hard time, well... let's just say it's lucky that I'm here to give Colin Morgan a hard time!
And, of course:
Bradley James: Have you been to Cardiff before?
Colin Morgan: Are there any ginger people in Cardiff?
Bradley James: ...I think there are one or two...
Colin Morgan: No, I'm not really looking forward to going to Cardiff.
Bradley James: Fair enough. The only acceptable form of racism, being that of racism against... gingers... apparently. I personally do not take part in such forms of racism.
Not to mention Bradley James 'translating' some French song lyrics as 'we're all going to die', Colin Morgan mouthing 'he bullies me every day! Help me!' at the camera, the spider incident, and 'je suis loser!' (which can be viewed here, starting at 07:50).
What about this one? This entire scene is priceless.
Bradley James: In a world that is... medieval... full of... medium-sized evil... one boy... has the power... to be... powerful...
And actually, the moment just before this is pretty darn hilarious too:
Bradley James: In a world that's medieval...wh...(looks up and sees a hot air balloon)...that's not very medieval...
THIS crowning moment of moments from Merlin: Secrets and Magic. Sing along now: "YOU'RE THE VOICE, TRY AND UNDERSTAND IT..."
The cast on the DVD commentaries also get some great one-liners in. In The Moment of Truth Katie is commenting on all the Ho Yay between Arthur and Merlin, leading to Bradley finally saying: "You're just fueling all those websites." Then you can hear Angel quietly say:
Angel Coulby: They don't need our help.
On the same track, half-way through the epic fight scene, Angel and Katie start gushing about one of the stunt-men that they found really hot.
Katie: Don't worry, I'll point him out again. You won't miss him.
Colin: (deadpan) Cheers.
In the commentary for The Once and Future Queen, Anthony Head has this to say about The Big Damn Kiss:
(to Angel) Aw, you look lovely there. (scene cuts to Bradley) He looks alright.
From Secrets and Magic for episode 1, Mackenzie Crook describes his fight with Merlin as "me and Merlin having a bit of rough and tumble in Arthur's chamber". He immediately realises what he's just said, and loses track of the interview for a few seconds.
All of the Outtakes. But highlights include Gwaine falling off a building into a pile of hay, Colin Morgan impersonating the Grinch and Anthony Head dissolving into helpless laughter. Several times.
And when Anthony Head accidentally calls Arthur "Bradley" whilst shooting one of the scenes.
The scene from The Fires of Idirsholas where Bradley pulls Anthony back off the desk and a piece of paper sticks to his face.
Gwaine tripping and falling flat while Merlin keeps walking and talking, not noticing Gwaine.
Bradley: If we're going to break everyone out, we must... remember our lines.
In the season 4 bloopers, Bradley and Colin flee down a corridor and into a chamber, slam the door behind them... and the door keeps going and swings open the other way. They laugh, and then shrug and keep running.
YMMV on the above; the appeal may be the fact that Bradley still seems dead serious as the door swings open before he adopts a grade-A Oh Crap expression, grabs Colin (who is on the verge of breaking character) by the arm and runs away while the latter laughs.
Colin Morgan randomly doing a cartwheel.
Nathaniel Parker taking over from Anthony Head as a helpless giggler. Honestly, the man cannot stop collapsing into giggles in every. single. scene!
The Knights of the Round Table and their complete inability to walk in a straight line without falling over. Truly they are Camelot's finest.
From Comic Con 2012, we have Season 5 bloopers.
Bradley drawing a dagger when he expected a sword. He looks at it, shrugs, and throws it away.
Eoin falling down while all the other knights run away. True Companions indeed.
While we're at it, his odd...er, gait when he runs. He's constantly tripping and yet he's not falling, so what do you call that?